1. Field of the Invention
This invention relates to a control device of a transmission fluid pressure for an electronically controlled automatic transmission wherein a shift shock in an automatic transmission for a vehicle is alleviated.
2. Discussion of Background
FIG. 1 is a block diagram showing a control device of an automatic transmission for a vehicle of the present invention, mentioned later, and a conventional example shown, for instance, in Japanese Unexamined Patent Publication No. 246653/1987. Explanation will be given to construction of a conventional example utilizing FIG. 1. In FIG. 1, a reference numeral 1 designates an engine, 2, an automatic transmission, 3, a rotational speed sensor for detecting either one of speeds of the engine 1 and the automatic transmission 2, 4, a control device for calculating a rotational speed change from a rotational number measured by the rotational speed sensor 3, and instructing a transmission fluid pressure based on the calculation, 5, a transmission fluid pressure controlling means for controlling a transmission fluid pressure in gear changing, 6, a pressure transfer means for applying the transmission fluid pressure which is instructed by the transmission fluid pressure controlling means, to friction elements of the automatic transmission 2.
Next, explanation will be given to the operation. When a gear ratio of the automatic transmission 2 is to be shifted, a rotational number, which is read from the rotational number sensor 3 of either the engine 1 and/or the automatic transmission 2, is inputted to the control device 4. Control device 4 which calculates a rotational number change by sampling the rotational number over time. Control device 4 takes the rotational number change from the read value, compares it with a target value, and instructs a transmission fluid pressure to the transmission pressure controlling means 5 based on the comparison result.
The instructed transmission fluid pressure is transferred to the friction elements such as a clutch or a brake of the automatic transmission 2 through the pressure transfer means 6 by the transmission fluid pressure controlling means 5, and performs engaging of the friction elements.
In this case, for instance, when the procedure of gear changing (e.g., changing from second gear to third gear) is fast, and the rotational number change is large, the control device decreases the transmission fluid pressure and thereby retards the speed of the engagement. Conversely, when the procedure of gear changing is slow and the rotational number change is small, the control device increases the transmission fluid pressure and thereby accelerates the speed of the engagement.
Explanation will be given to the operation utilizing FIGS. 4 and 5. FIG. 4 is a flow chart showing an operational flow of a conventional control device of an automatic transmission, and FIG. 5, a change of a rotational number in the gear changing of a turbine shaft which is one of rotational bodies of the automatic transmission 2.
First, in step S1, the operation calculates a vehicle speed by respective sensors installed on a vehicle. In step S2, the operation reads a throttle opening degree from a throttle sensor. In step S3, the operation calculates a turbine shaft rotational number and a turbine shaft rotational number change. In step S4, the operation determines whether the engine is in gear changing by means of the control device 4. When the operation determines that the engine is in gear changing, as a result of the determination, the operations jumps to step S7 from Y side of step S4.
Furthermore, when the operation determines that the engine is not in gear changing in step S4, the operation proceeds to step S5 from N side of step S4. In step S5, the operation determines whether the engine is to perform the gear changing or not. When the engine is not is to perform the gear changing, the operation returns to step S1 from N side of step S5. Conversely, when the engine is to perform the gear changing in step S5, that is, when the engine starts the gear changing, the operation proceeds to step S6. In step S6, the operation switches shift steps, and proceeds to step S7.
In step S7, the operation performs a treatment in case that the operation determines that the engine is in the gear changing, or that the engine is to perform the gear changing (begins to perform the gear changing) in step S5. After the operation switches the shift steps in step S6, the operation calculates a target turbine rotational number change from a vehicle speed or the like, and compares it with the turbine shaft rotational number change obtained in step S3. In step S8, the operation outputs a clutch/brake transmission fluid pressure in the gear changing by performing a negative feed back control. In step S9, when the gear changing is not finished, the operation returns to step S1. When the gear changing is finished, the operation performs a gear changing finish treatment in step S10.
At this point, a more detailed explanation will be given as to the negative feed back control process and the role of the clutch/brake fluid pressure in the gear changing. In FIG. 5, in an engine state wherein the gear changing is proceeding, a target turbine rotational number change 17, is calculated, and the actual turbine rotational number change determined. The two valves are compared. If the actual turbine rotational number change is shifted to a direction 18 which is inclined more gently, the control device controls the actual turbine rotational number change to the target turbine rotational number change 19 by increasing the transmission fluid pressure. Conversely, when the actual turbine rotational number change is shifted to a direction 20 which is inclined more steeply, the control device makes the actual turbine rotational number change to the target turbine rotation number change 21 by decreasing the transmission fluid pressure, to thereby retard the gear changing. Because fluid pressure affects the clutch, the clutch affects the planetary gear, and the planetary gear in turn affects turbine rotation, it is in this fashion that the negative feed back is performed.
Since the conventional control device of the automatic transmission for an automobile is constructed as above, as shown in FIG. 6, when the engine shifts from a state of POWER OFF wherein the engine is in a deceleration/closed throttle mode during which the engine functions as a dynamic brake to a state of POWER ON when the engine is in a constant speed or acceleration mode the turbine shaft rotational number begins to increase after a time lag (point b). By this increase the transmission fluid pressure rapidly increases since the negative feed back control is operated. After a certain time, the fluid pressure surpasses a speed increasing power of the engine, and the turbine shaft rotational speed change rapidly changes, thereby generating a shock at point c.